People keep missing the point that working at a championship club isn't supposed to make him ready for United. It's supposed to make him ready for the next step on the journey to being good enough to United, if he has it in him. If he does the job Eddie Howe has done, and being Ryan Giggs, he'd get a bigger club than Bournemouth, why can't he be picked up by a Stoke, or a West Brom? That could lead to an Everton, which could lead to Manchester United. That's just an example of a possible route. If he does really well at one or two clubs instead of 3/4, and our team is in a good place, we might go for him early.
Sir Alex didn't start at United or even Aberdeen. Mourinho, while he did begin with a brief 9 game spell at Benfica did have to manage Uniao de Leiria before Porto.
This is SAF showing early promise at St Mirren (first job)
Mourino at that other club
This is how I see it, and I have been saying the same for some time. Even if he is flawless in a shit championship team, that shouldn't give him the United job, but instead that would give him a job in EPL. Then do then well and get the Spurs or Evertons of this world. And if he continues doing well there, then he gets United (or a similar team, I mean strange things have happened, Sir Matt Busby was a captain of Liverpool after all).
Of course, Giggs might take shortcuts which might backfire (see Gary Neville), or United might give him the job in lets say by doing well on 'whatever club' in Championship and Stoke in EPL, instead of 'whatever', Stoke and Everton, especially if we are in a strong position.
But from Championship to United, it shouldn't happen. The entire point is to do well and get a better job, not do well and get immediately the top job. Take Diego Simeone for example, Atletico is his seventh job as a manager, and Atletico aren't an elite club. SAF started on Scotland's third league, and promoted that team twice before he got the Aberdeen job, and only after a very good job there when he won the UEFA Cup, he got the United job (and United wasn't near as big as now). As you said Mourinho's first real job was at Uniao, not Porto. Ancelotti managed Regianna and Parma before he got Juve and Milano.
As I see it, it is either an incremental procedure, or an immediate promotion to the job. I was strongly against the latter, and Ed seems to have decided against it. Now the ball is on Giggs' side to prove that we were wrong to not give him the job and I wish him all the best in his career. I would be very happy to see him at United in the future, getting the job on merit, and I would be happy to see him managing a top club in the future if United job never happens. However, if he is really good at it, then there is no reason why we won't give him the job. But only then, when he proves that he is a top manager.